Such a vehicle as an automobile has a power supply unit mounted therein for supplying power to a plurality of loads. Well known as this power supply unit is, for example, what is illustrated in FIG. 14. As shown in the figure, the power supply unit 100 is provided with a battery 101, ramp loads 102 as a plurality of loads, and a controller 103 disposed between the battery 101 and the plurality of ramps 102.
The aforementioned controller 103 is mounted to, for example, a power supply box disposed near the battery 101. This controller 103 incorporates a branch circuit 103a branching a power line form the battery 101 into a plurality of branch lines, and a plurality of switch elements 103b each disposed on the branch lines branched by the branch circuit 103a. A Mechanical relay or transistor relay is used as the switch elements 103b. The switch elements 103b is controlled to switch on or off by supplying a drive signal from a main drive device 200.
According to the power supply unit 100 shown in FIG. 14, to switch on the switch elements 103b by the main drive device 200 allows power from the battery 101 to be supplied to the ramp loads 102, and to switch off the switch elements 103b by the main drive device 200 allows power supplied from the battery 101 to the ramp loads 102 to be shut down.
The automobile, in order to charge the battery 101, also has alternate 104 mounted thereto as a generator transforming mechanical energy of an engine into an electric energy. In order to supply power this alternate 104 generates directly to the ramp loads 102, this alternate 104 may be disposed between the battery 101 and the ramp loads 102. For example, with the battery 101 in 48V-system and with the ramp loads 102 in 12V-system, a DC/DC convertor 105 may be disposed between the battery 101 and the ramp loads 102 that steps down the power voltage from the battery 101.
A cable L01 that connects between the battery 101 and the controller 103 is required to apply large current to which currents applied to the plurality of ramp loads 102 total up, and thus is applied for the one in which its cable size is large and pathway resistance is small. On the other hand, the cable L11 connecting the controller 103 and each ramp load 102 may apply current only applied to ramp loads 102 to be connected, and thus is applied for the one in which its cable size is small and pathway resistance is large.
Therefore, mounting the aforementioned controller 103 in the power supply box near the battery 101 induces the cable L11 with larger pathway resistance than the cable L10 with small pathway resistance to become long, increasing voltage drop caused by the cable L11. Therefore, drawbacks were posed that power loss caused by the cable L11 becomes large and thus voltage applying to the ramp loads 102 becomes low.
Furthermore, the aforementioned ramp loads 102 is regulated in its rated voltage, and when voltage beyond the rated voltage is applied, power the ramp loads 102 consume increases so as to become bright beyond necessity. It follows from this that applying voltage beyond the rated voltage to the ramp loads 102 results in applying useless voltage. Furthermore, the higher the voltage applied to the ramp loads 102, the more electrical stress applied to the ramp loads 102, possibly deteriorating product life (it is exactly well known that the ramp loads 102 such as filament when beyond the rated voltage becomes short in their product life).
Then, it is thought that voltage outputted from the alternate 104 or DC/DC convertor 105 is dropped to the rated voltage of the ramp loads 102 so as to become unwasted. The voltage, however, cannot be dropped to the rated voltage outputted from the alternator 104. This is attributed to the fact that in the automobile power is regenerated for improvement of fuel efficiency in which the battery 101 is charged by the alternator 104. In this power regeneration, for example, with the voltage of the battery 101 in 12 volt, it is required that the voltage outputted from such the alternator 104 is set to 14 volt higher than 12 volt. Some automobile may also output voltage higher than 14 volt. Therefore, dropping the voltage outputted from such the alternator 104 to the rated voltage disenables to charge the battery 101.